A considerable amount of oil present in geological formations is stored in heterogeneous reservoirs having a heterogeneous permeability distribution. Such heterogeneous reservoirs can be fractured reservoirs where oil is contained in porous rock blocks, also referred to as reservoir matrices, which are separated by a network of fractures. The permeability of fractures can exceed the permeability of the rocks matrices by several orders of magnitude. In addition the permeability of the porous rocks can vary locally, e.g. due to different sizes of pores and pore throats. Rock regions having large pore throats usually have a higher permeability compared to the regions having small pore throats. Fractured reservoirs can be characterized by the parameters of wettability of the rock matrices, fracture apertures, and fracture configuration.
Conventional methods for oil recovery from geological reservoirs include displacement methods by water flooding or gas flooding. These methods have in common that a chase fluid, which can for example be water, a solvent or a gas, is injected into a reservoir through an injection well in order to displace oil from the reservoir, and drive the oil towards distant production wells. Depending on the choice of the chase fluid, its viscosity, temperature conditions, and pressure exerted onto the chase fluid, the displacement mechanism can either be miscible displacement, immiscible displacement, also referred to as viscous displacement or diffusion.
Generally, when a chase fluid is injected into heterogeneous reservoirs under pressure, it will preferably propagate through the reservoir along the path of highest permeability. This is especially the case for chase fluids having a low viscosity compared to the oil to be recovered, such as gas. Therefore, viscous chase fluids are preferred for displacement methods under pressure. Nevertheless, in a fractured reservoir or heterogeneous reservoirs in general, fractures and highly permeable streaks will form main passages for injected fluids. This leads to a reduced efficiency in displacement recovery from heterogeneous reservoirs.
For increasing the efficiency of displacement recovery from heterogeneous reservoirs, it is known to plug the highly permeable streaks, such that the chase fluid will enter into the less permeable rock blocks in order to displace oil from the blocks. Known methods for plugging of fractures and highly permeable streaks include injection of chemicals, such as disclosed for example in U.S. Pat. No. 3,882,938, or generation of foam in the permeable streaks, the latter of which especially reduces the mobility of gas as a chase fluid. Consequently, the efficiency of CO2-gas flooding can be increased by previous generation of CO2 containing foams in the reservoir.
Application of pressure on the chase fluid in heterogeneous reservoirs suffers from the additional drawback, that when the chase fluid and oil propagates through the rock matrices under pressure, usually the differential driving pressure can not be transmitted from one block to the next adjacent block across a fracture due to lack of capillary contact. This leads to a propagation of the chase fluid preferentially in the fractures, or depending on the wettability conditions of the matrices, on a block by block basis, relying on spontaneous imbibition of the wetting phase for the oil recovery from the matrix blocks and thus a loss of differential pressure across the blocks, causing low oil recovery.